The force generated by a "propulsion-engine", (a device for generating a force for propelling a vehicle through a fluid medium), can operate using forces derived from two different physical effects, but leading to the same overall result. This can be illustrated using two simple examples.
Suppose a man is in a boat. He now takes a bucket and dips some water from the side of the boat and throws the water from the bucket in a rearward direction. The act of throwing exerts a force on the bucket, (man-boat system), due to "reaction" in delivering "momentum" to the water. This is the principle employed in current propeller and jet-boat propulsion systems.
In our second example, which forms the basis of the present invention, suppose that we had a pipe mounted under the water and closed at both ends forming a chamber. We now evacuate some of the air from the chamber. Note that the atmospheric pressure minus the internal pressure, will be pushing against each end of the chamber in equal and opposite directions. There will be no net thrust on the chamber. At some time t.sub.1 we open a port, (a bit smaller in diameter than the pipe), at the front of the pipe. Water will be forced into the pipe by the atmospheric pressure in a flying jet into the partial-vacuum. Well before the front of the flying jet of water reaches the other end, we close the intake port and allow the slug of water to continue down the pipe through the partial vacuum. Just before it gets to the end we admit air into the pipe to bring it back to the pressure outside the pipe, and open the back end of the pipe to let the water fly out. Note that in this case, the actual force pushing against the chamber is the water pressure pushing against the back of the pipe minus the differential pressure across the intake orifice, times the orifice area. If we are at zero speed with respect to the water this will be simply the pressure in the water minus the inside pressure, times the orifice area.
For moving cases, the calculation is more complex. The "stagnation" pressure of the water with respect to the moving boat is P.sub.s =1/2.rho.V.sub.B.sup.2, (the differential pressure required to squirt a jet of water out of a hole at a velocity V.sub.B). To this we add the differential pressure (p.sub.o -p). This yields the effective pressure differential. The velocity of the incoming water with respect to the boat is then ##EQU1## where .rho. is the density of the water.
The mass-flow-rate of the incoming water dm/dt=A.rho.V.sub.wj Kg/sec. and the force is the mass rate times the velocity of the jet with respect to the static water. ##EQU2## Thus while the actual force is due to the differential-pressure on the back of the chamber, we compute it using the water-jet momentum relationship.
It is well known that the propulsion efficiency is highest for a system which ejects the "working water", i.e. the fluid moved by the propulsion system with respect to the "rest" water, at the lowest possible velocity with respect to the water through which the boat is moving, (the "rest" coordinate system). For an "ideal" system, the propulsion efficiency, (ratio of power provided by the system to thrust the boat divided by the power from the engine), is given by the simple equation: ##EQU3## Here V.sub.B is the boat velocity with respect to the rest water, and V.sub.wj is the velocity of the water jet with respect to the boat.
The thrust of the propulsion system as we have seen, is the product of the mass rate of flow of the working water involved in the propulsion system, times the velocity of the ejected water with respect to the rest system. Clearly systems with high mass-flow rate and low velocity-differential are desirable from a "propulsion efficiency" point of view.
When a fluid such as water is constrained to flow inside a tube, the walls of the tube tend to impede the flow through "surface friction". The most commonly used mathematical formulation of this effect is the head-loss due to frictional flow. This is given by the formula: ##EQU4##
In current jet-boats, the fluid velocity is very high through the pumping system, and the corresponding losses due to friction and turbulence i.e. "flow losses", are major detriments to efficient operation.
For smooth pipes the parameter f is of the order of 0.01 to 0.02 depending on the Reynolds number of the system. L is the length of the pipe and D is the pipe "hydraulic" diameter.
In addition to such duct frictional losses, the high specific speed pumps used is current jet boats are low in "pumping efficiency" due to the other frictional losses associated with the swirling motions inherent in the pumping means involved.
Propeller driven systems induce large vortex motions in the water with attendant energy losses. Propellers on small boats generally operate at high peripheral speed approaching cavitation velocities. This results in large surface-friction losses. In addition there is a drag due to the propeller effective cross-sectional area which is also a major loss-factor.
The Significant Technical Advantages of this Invention are:
(1) Large mass-flow and low relative-velocity flow in an "internal jet" propulsion system yielding high "propulsion efficiency".
(2) Minimization of the internal-flow-losses in the system by having virtual non-contact flow of the internal water jet through the pumping means.
(3) A high pump energy-efficiency, through the use of air, (rather than water), as the working fluid in the mechanical pump. The work in this system is done by the air-pump.